WO-A-01/98393 describes 2K (2-component) coating compositions comprising a polyol binder component and a polyisocyanate crosslinker component partly functionalized with alkoxysilyl groups. These coating compositions are used as primers and are optimized for adhesion to metallic substrates, especially aluminum substrates. Over these coating compositions, as part of an OEM finish or a refinish, it is possible to apply basecoat/clearcoat systems. In terms of scratch resistance and weathering stability, the coating compositions of WO 01/98393 are not optimized.
EP-A-0 994 117 describes moisture-curable mixtures comprising a polyol component and a polyisocyanate component which may partly have been reacted with a monoalkoxysilylalkylamine that has undergone reaction preferably to an aspartate. Although coatings formed from such mixtures do have a certain hardness, they are nevertheless of only limited suitability for OEM applications in terms of their weathering stability and, in particular, their scratch resistance.
US-A-2006/0217472 describes coating compositions which can comprise a hydroxy-functional acrylate, a low molecular mass polyol component, a polyisocyanate, and an amino-functional alkoxysilyl component, preferably bisalkoxysilylamine. Such coating compositions are used as clearcoat material in basecoat/clearcoat systems and lead to scratchproof coatings. Coating compositions of this kind, however, have only very limited storage qualities, and the resulting coatings have low weathering stability, particularly with respect to UV radiation in a wet/dry cycle.
WO 2006/042585 describes clearcoat materials which are suitable for OEM finishing and which as their main binder component comprise polyisocyanates whose isocyanate groups, preferably to an extent of more than 90 mol %, have undergone reaction with bisalkoxysilylamines. Clearcoat materials of this kind combine excellent scratch resistance with high chemical and weathering resistance. But there is still a need for a further improvement in the weathering stability, particularly with respect to cracking under UV irradiation in a wet/dry cycle, with retention of the high level of scratchproofing.
EP-A-1 273 640 describes 2K coating compositions composed of a polyol component and of a crosslinker component consisting of aliphatic and/or cycloaliphatic polyisocyanates, 0.1 to 95 mol % of the free isocyanate groups originally present having undergone reaction with bisalkoxysilylamine. These coating compositions can be used for OEM finishing and when fully cured combine good scratch resistance with effective resistance to environmental influences. Nevertheless, these coating compositions have a particularly strong propensity toward aftercrosslinking, with the consequence—directly after thermal curing to completion—of inadequate scratch resistance of the coatings. The significant after crosslinking likewise impacts adversely on the weathering stability, since there is an increased risk of stress cracks.
In the case of coating materials which crosslink simultaneously through the formation of urethane bonds and the formation of siloxane bridges, the problem arises that the hydrolyzable silane groups are very reactive with respect to water. This may lead, for example, in the case of the pneumatic application of these coating materials, to disruptive secondary reactions on the basis of the atmospheric humidity. In the process, gel particles may form which adversely affect the quality of the resulting film coatings.
It was an object of the present invention to provide coating compositions, particularly for the clearcoat film in OEM finishes and automotive refinishes, that lead to a network with a high degree of weathering stability, the unwanted formation of moieties unstable to hydrolysis and weathering being very largely suppressed, in order to ensure high acid resistance. In addition, the coating compositions ought to lead to coatings which have a high degree of scratchproofing directly after thermal curing, and in particular a high retention of gloss after scratch exposure. Moreover, the coatings and coating systems, especially the clearcoat systems, ought to be able to be produced even in film thicknesses>40 μm without stress cracks occurring. This is a key requirement for the use of the coatings and coating systems, particularly of the clearcoat systems, in the technologically and esthetically particularly demanding field of automotive OEM finishing.
The intention in particular was to provide clearcoat systems featuring high resistance, particularly to cracking, under weathering with UV radiation in a wet/dry cycle, in combination with outstanding scratchproofing.
Furthermore, the new coating compositions ought to be preparable easily and with very good reproducibility, and ought not to present any environmental problems during application of the coating material.
A key problem facing the present invention is to prevent hydrolysis of the silane groups by the influence of atmospheric humidity during the application for coating materials which are cured by hydrolysis of alkoxysilane compounds and, additionally, by the reaction of isocyanate groups with hydroxyl groups. Surprisingly it has been found that this problem can be solved through the addition to the coating compositions of a compound which is very unstable to hydrolysis. The reaction of this compound proceeds more rapidly than the silane hydrolysis and thereby suppresses the unwanted formation of gel particles.